1. Field of the Invention
Embodiments of the present invention generally relate to the deposition of thin films. More particularly, this invention relates to a process and apparatus for depositing a thin film onto a substrate surface and conditioning the substrate surface.
2. Description of the Related Art
Developing environmentally friendly energy sources have gained significant interest recently in various industries related to the generation of power and electricity. Various types of fuel cells can be used to directly produce electricity for a number of applications, such as portable electronics, cell phones, wireless devices, PDAs, cameras, portable players, computer notebooks, mobile vehicles (e.g., cars, trucks, trains, etc.), stationary large size energy equipment, residential electricity and others. Since semiconductor machining technology can be easily utilized in fuel cell manufacturing, efficient production of electricity by fuel cells is feasible.
A fuel cell is an electrochemical device in which a gaseous or liquid fuel reacts with an oxidant to produce electricity. Generally, an electrolyte is sandwiched by two electrodes, an anode and a cathode, to form a fuel cell unit. A fuel, such as pure hydrogen or hydrogen reformed from any hydrocarbon fuel, is fed into the anode to be oxidized into a proton and an electron. An oxidant, such as air or oxygen, is flowed into the cathode to react with the proton which has passed through the electrolyte and, in some cases, through a proton-permeable membrane. The electron forms a separate current that can be utilized to generate electricity before returning to the cathode to be reunited with the proton and the oxidant, resulting in by-products, such as heat and water. Each fuel cell unit is stacked or arranged together to form a fuel cell stack or module. A number of modules or fuel cell stacks are piled, and electrical terminals, electrical insulators and end plates are disposed at opposite ends of the pile of modules to collectively produce electricity.
The essentials of a fuel cell are generally simple, leading to highly reliable and long-lasting electricity/energy generating applications. Fuel cells are also highly efficient, converting hydrogen fuel into useful energy at an efficiency rate as high as 60 percent, as compared to about 35 percent for combustion gas engines or alkaline batteries. Further, the by-product of the main fuel cell reaction, when hydrogen is the fuel, is pure water, which means the carbon dioxide (CO2) emission of a fuel cell is low and can be essentially zero emission. Also, fuel cells are very quiet, even those with extra equipments (e.g., fuel pump, air pump, and thermal control systems, etc.), making them suitable for both portable power applications and for local power generation.
Generally, a fuel cell stack uses a number of conductive plates, placed between adjacent fuel cells in a fuel cell stack to separate each fuel cell. The conductive plates usually incorporate flow channels or grooves for feeding and moving any fuel gas, oxidant or fluid through the fuel cell. The conductive plates may be made of a metal or a conductive polymer, such as a carbon-filled composite. Each conductive plate includes one side for flowing fuel gases or oxidant gases. The other side of the conductive plate generally contains cooling channels or conduits, which are mated with the cooling channels from an adjacent fuel cell in a fuel cell stack to form into a mated conductive plate having an internal cylindrical path for flowing coolants to move the heat and water produced from the chemical reactions at an anode and/or a cathode away from the fuel cell stack. A mated conductive plate thus includes one side to serve as an anode for one fuel cell and the other side to act as a cathode for an adjacent fuel cell. Therefore, the conductive plates placed between adjacent fuel cells in a fuel cell stack are also called a bipolar plate or a separator plate and the conductive plates placed at both ends of a fuel cell stack are also called end plates.
The electrolyte plays a key role in a fuel cell to carry protons from one electrode, the anode, to the other electrode the cathode. The electrolyte includes various types of organic and inorganic chemicals and, thus, different types of fuel cells are formed depending on the types of chemicals used. In addition, the electrolyte may include a membrane, such as a polymer membrane for a direct methanol fuel cell (DMFC) or an immobilized liquid molten carbonate for a molten carbonate fuel cell (MCFC). One type of fuel cell, a proton exchange membrane fuel cell (PEMFC), uses a thin proton exchange membrane with both sides of the surfaces coated with different catalysts, which accelerates the different chemical reactions at the anode and the cathode. The membrane is sandwiched by two microporous conductive layers (which function as the gas diffusion layers and current collectors) to separate the hydrogen fuel from the anode and the oxidant from the cathode and thus forming a membrane electrode assembly (MEA).
The MEA must permit only the proton to pass between the anode and the cathode. If free electrons or other substances travel through the MEA, they would disrupt the chemical reactions and short circuit part of the current. Further, in order for a fuel cell to operate properly with high electrical output and reliability, the gas and fluids must be moved through the surface of parts, channels, conduits, passages, grooves and/or holes inside the fuel cell without interruption under a wide variety of operating conditions. As such, the surface properties of any fuel cell parts must be conditioned to facilitate and enable this movement. In addition, various parts of a fuel cell stack or module should provide a surface with good contact to electrolyte, current or any gas, fluid present in the fuel cell stack.
For example, the surface of fuel cell parts should provide a low fluid contact angle to the electrolyte as well as high conductivity for current to go through. The surfaces of fuel cell parts may need to be conditioned to provide good electrical contact such that contact landing areas may not need to be masked. In addition, the surface of fuel cell parts should withstand by-product heat and cycles of temperature variation between low temperatures (e.g., −40° C.) and high temperatures (e.g., 100° C.). Furthermore, conditioning the surfaces of fuel cell parts can protect the parts from the attack of acid, water (H2O), oxygen (O2) and any other chemicals in the electrolyte to ensure long part lifetime and long term reliable operation. Surfaces conditioned by a thin film or coating should be resistant to peeling or swelling inside the fuel cell environment. Also, an acidic environment in the fuel cell may be created from the release of fluorine present in the membrane electrode assembly (MEA), a fuel cell part may need to be conditioned to sustain etching by such a hydrofluoric acid (HF)-containing environment.
Thus, there is still a need for methods and apparatus for conditioning a surface of a substrate, such as a fuel cell part, with improved fluid contact angle and low acid etch rate to protect the substrate.